A plastic optical fiber has hitherto been used in short-distance transmission systems by virtue of its advantages, such as superiority to a quartz glass fiber in flexibility, even in the case of a large aperture, low loss when connected to a light source because an optical fiber having a light weight and a high numerical aperture can be easily prepared therefrom, and very low cost because it can be massproduced on a commercial scale. However, conventional optical fibers have a drawback in that their transmission loss remarkably increases at a service temperature of 80.degree. C. or above, which limits their application from the viewpoint of the serviceable temperature when the optical fiber is used for automobiles, aircraft, marine vessels, etc. Therefore, a polycarbonate of bisphenol A having a high glass transition temperature and flame retardancy has been used in the above-described applications. However, current polycarbonates cause birefringence when a laser beam is passed through the fiber mainly because of thermal stress, molecular orientation, and residual stress due to change in the volume around the glass transition point caused during the steps of cooling and flowing a resin in the molding of a fiber. An increase in transmission loss due to large optical heterogeneity caused by briefringence is a fatal drawback of a plastic optical fiber.